1. Field of the Invention
The present invention relates to a developing device that forms a toner image by supplying a magnetic developing agent toward an image carrier on which an electrostatic latent image is formed by an electrophotographic method, and an image forming apparatus equipped with the developing device.
2. Description of the Related Art
A developing device applied to an image forming apparatus adopting an electrophotographic method forms a toner image by supplying toner particles as a developing agent toward the peripheral surface of the photoconductive drum on which an electrostatic latent image is formed according to image data. The image forming apparatus equipped with such a developing device transfers the toner image formed on the photoconductive drum onto a recording medium, such as a sheet. The transferred toner image is fixed onto the recording medium by heating and applying a pressure to the recording medium by a fixing device provided downstream of the photoconductive drum in the sheet transportation direction. By these operations, the image forming apparatus forms an image according to the image data on the recording medium.
The developing device is provided with a rotary sleeve to supply toner particles to the photoconductive drum, and a blade is provided at a position upstream of the position at which the rotary sleeve and the photoconductive drum oppose each other in the rotation direction of the rotary sleeve. By regulating the thickness of a toner layer that is formed thick on the rotary sleeve with the blade, it is possible to prevent an excessive supply of toner particles to the photoconductive drum. The developing device configured in this manner is therefore able to supply toner particles in a homogeneous state to the photoconductive drum via the rotary sleeve.
As a developing device using a magnetic mono-component developing agent (magnetic toner particles), there is known, for example, a magnetic mono-component developing device that regulates the thickness of a toner layer made up of magnetic toner particles on the rotary sleeve accommodating a stationary magnet using a non-contact magnetic blade. In such a magnetic mono-component developing device, when the toner layer formed thick on the rotary sleeve passes by the position at which the rotary sleeve and the magnetic blade oppose each other (hereinafter, referred to as the layer thickness regulating position), the toner layer is divided in two between the rotary sleeve and the magnetic blade. Toner particles closer to the magnetic blade adhere to the magnetic blade and toner particles closer to the rotary sleeve move away from the magnetic blade as they are attracted to the rotary sleeve by a magnetic attraction force of the stationary magnet accommodated in the rotary sleeve. In this manner, the magnetic mono-component developing device regulates the thickness of the toner layer without the magnetic blade contacting with the rotary sleeve instead of scarping off the toner layer using the blade. Accordingly, the magnetic mono-component developing device has minor damage on the magnetic toner particles, the blade, and the rotary sleeve even when high-speed printing is executed, and is therefore suitably applied to an image forming apparatus that requires high-speed printing and high durability.
However, because the magnetic mono-component developing device regulates the thickness of the toner layer with the magnetic attraction force of the stationary magnet accommodated in the rotary sleeve or the like, it is susceptible to the surrounding environment, such as the charging property of toner particles, free fine toner particles, an external additive to toner particles, and the surface nature of the rotary sleeve. Accordingly, disturbance may possibly occur in the toner layer whose layer thickness has been regulated, and the toner layer formed under such a condition may have irregularities in density.
As a developing device that reduces the occurrence of irregularities in density in the toner layer as above, there is known a developing device as described in JP-A-2003-167426 (Reference D1), in which a magnet is attached to the surface of the blade on the side upstream in the rotation direction of the rotary sleeve.
The developing device described in Reference D1 is able to regulate the thickness of the toner layer without disturbing the toner layer when a strong magnetic field is generated at the end surface of the blade on the rotary sleeve side by the magnet attached to the blade, and a homogeneous toner layer having little irregularities in density can be obtained. However, depending on the location of the magnet attached to the blade, there may be a case where a strong magnetic field is also generated on the end surface of the magnet on the rotary sleeve side. In such a case, a large amount of magnetic toner particles adhere to the end surface of the magnet on the rotary sleeve side. In a case where the developing device is used over a long period or at a high temperature in this state, toner particles adhering to the magnet aggregate to form clusters. When such aggregated toner particles enter at the layer thickness regulating position, a part mixed with the aggregate toner particles is in a condition that the thickness and the charging state of the toner layer are different from those in the other parts. This gives rise to a streak-like image defect.